The invention concerns a calender, in particular a supercalender, on whose frame a set of rolls is mounted, which is shaped as a stick of rolls placed one above the other and which comprises an upper roll, a lower roll and several intermediate rolls placed between the upper roll and the lower roll, the rolls being supported on the frame by the intermediate of the base parts as vertically displaceable along guides provided in the frame, of which at least the base parts of the intermediate rolls can be positioned in the vertical direction by means of lifting spindles provided in the frame and by means of spindle nuts provided on the spindle.
The system of rolls in a conventional supercalender comprises a number of rolls, which are arranged one above the other as a stack of rolls. The rolls placed one above the other are in nip contact with each other, and the paper web to be calendered is arranged to run through the nips between the rolls. The rolls in the system of rolls are normally mounted rotatably in bearing housings, which are again attached to base parts that are fitted to glide on vertical guides provided in the frame of the calender. Moreover, the base parts are provided with stop parts, which are fitted on vertical lifting spindles provided in the frame of the calender. Thus, one of the functions of the lifting spindles is to act as guides so as to keep the rolls in the system of rolls in the correct position. Thus, the bearing housings of the rolls in the system of rolls are not fixed rigidly to the calender frame, but the bearing housings, and consequently also the rolls, can move vertically. Since the masses of the bearing housings of the rolls and the auxiliary devices attached to the housings are quite large, in conventional supercalenders this causes the considerable drawback that these masses of the bearing housings and of the auxiliary devices attached to the bearing housings cause distortions in the distributions of the linear loads in the nips Thus, the linear load in the nips is not uniform, but it is substantially higher at the ends of the nips than at the middle. Since in the systems of rolls of supercalenders there are several rolls placed one above the other, as was already stated above, this further results in the linear loads in individual nips being cumulated and causes a considerably large error in the overall linear load. This defective distribution of the linear load deteriorates the quality of the calendered paper.
With a view to solving the problem described above, in the Applicant's earlier FI patent application No. 880137 it is suggested that the system of rolls be provided with lightening devices, which are supported on the base parts of the rolls, on one hand, and on the spindle nuts provided on the lifting spindles, on the other hand, so that by means of these lightening devices, the distortions caused by the weight of the bearing housings and of auxiliary devices attached to the housings, e.g. takeoff rolls, in the lateral areas of the profiles of linear loads between the rolls can be eliminated. Also, for conventional machine calenders, a solution is known in the prior art wherein the rolls of the machine calender are provided with a lightening system, in particular with hydraulic lightening cylinders for elimination of concentrated loads arising from the bearing housings of the rolls and from auxiliary devices. It is a simple matter to provide machine calenders with such relief devices, because the rolls in the system of rolls of a machine calender are mounted on the frame of the calender by the intermediate use of levers with articulated joints. It is, however, quite difficult to use devices corresponding to the machine calenders in supercalenders because of the constantly varying diameters of the fiber rolls and because of the large number of rolls in supercalenders.
Owing to their construction, which was described above, conventional supercalenders also have a further drawback, which is concerned with the vertical movement of the rolls in the system of rolls. As was already explained above, the bearing housings of the rolls in the system of rolls are mounted on base parts, which is vertically mobile along glides provided in the frame of the calender. This further drawback is related to the friction at the guides, which is effective between said guides and the base parts. Under these circumstances, owing to the friction at the guides, the rolls in the system of rolls cannot move or be positioned in the vertical direction completely freely, which may cause disturbances in the operation of the calender as well as considerable local errors in the distributions of the linear loads. With a view to eliminating the frictions at the guides, in supercalenders it would be possible to think of using the solution described above, commonly known from machine calenders, wherein the rolls are mounted on the calender frame by the intermediate use of lever systems provided with articulated joints. The use of such an arrangement in supercalenders is, however, limited by the fact that the system of rolls in a supercalender includes several fiber rolls, whose diameter may vary to a considerable extent. Thereby, owing to the variations in the diameters of the rolls, the rolls must be able to move vertically to a considerable extent. If the rolls were mounted to the frame of the calender by the intermediate structure of lever systems with articulated joints, in such a case a vertical shifting of the rolls would also cause a considerable shifting in the transverse direction.